Replacing 1500 bytes at a variable offset

[A. Farber]

How could I replace 1500 bytes in a file, please?
They are not located at some fixed offset and also
the amount of replacing bytes could be bigger or
smaller than 1500. Here's what I am trying currently:

my $SEARCH = pack 'C*', map { hex } qw(
98 e6 52 ca db ce db 19 62 00 00 00
69 b5 40 0c 91 dc 6b 43 cf 00 00 00
5b 7d 92 c2 e1 ec f1 45 11 00 00 00
92 45 a3 64 0b ff e4 d9 a6 00 00 00
2e 0d cb 22 50 0e 92 5a 0f 00 00 00
4e d5 78 59 ee 1c 8b c6 cc 00 00 00
.... );

my $REPLACE = pack 'C*', map { hex } qw(
ff ff ff ff ff ff ff ff ff ff ff ff
69 b5 40 0c 91 dc 6b 43 cf 00 00 00
5b 7d 92 c2 e1 ec f1 45 11 00 00 00
92 45 a3 64 0b ff e4 d9 a6 00 00 00
2e 0d cb 22 50 0e 92 5a 0f 00 00 00
.... );

use constant KEYSIZE => length $SEARCH;

while (sysread $upload, $chunk, KEYSIZE) {
$both = $prev . $chunk;
if ($both =~ s/$SEARCH/$REPLACE/o) {
die "syswrite failed: $!"
unless length $both == syswrite $fh, $both;
undef $prev;
} else {
die "syswrite failed: $!"
unless length $prev == syswrite $fh, $prev;
$prev = $chunk;
}
}
# don't forget to print the last line
die "syswrite failed: $!"
unless length $prev == syswrite $fh, $prev;

Unfortunately the regex doesn't compile, probably
because it doesn't like the binary data I feed to it!

Regards
Alex

 

[John W. Krahn]

How could I replace 1500 bytes in a file, please?
They are not located at some fixed offset and also
the amount of replacing bytes could be bigger or
smaller than 1500. Here's what I am trying currently:

my $SEARCH = pack 'C*', map { hex } qw(
98 e6 52 ca db ce db 19 62 00 00 00
69 b5 40 0c 91 dc 6b 43 cf 00 00 00
5b 7d 92 c2 e1 ec f1 45 11 00 00 00
92 45 a3 64 0b ff e4 d9 a6 00 00 00
2e 0d cb 22 50 0e 92 5a 0f 00 00 00
4e d5 78 59 ee 1c 8b c6 cc 00 00 00
.... );

my $REPLACE = pack 'C*', map { hex } qw(
ff ff ff ff ff ff ff ff ff ff ff ff
69 b5 40 0c 91 dc 6b 43 cf 00 00 00
5b 7d 92 c2 e1 ec f1 45 11 00 00 00
92 45 a3 64 0b ff e4 d9 a6 00 00 00
2e 0d cb 22 50 0e 92 5a 0f 00 00 00
.... );

use constant KEYSIZE => length $SEARCH;

while (sysread $upload, $chunk, KEYSIZE) {
$both = $prev . $chunk;
if ($both =~ s/$SEARCH/$REPLACE/o) {
die "syswrite failed: $!"
unless length $both == syswrite $fh, $both;
undef $prev;
} else {
die "syswrite failed: $!"
unless length $prev == syswrite $fh, $prev;
$prev = $chunk;
}
}
# don't forget to print the last line
die "syswrite failed: $!"
unless length $prev == syswrite $fh, $prev;

Unfortunately the regex doesn't compile, probably
because it doesn't like the binary data I feed to it!

It is not that the data is binary, it is because some of the characters
are special to regular expressions. Use the quotemeta escape to fix it.

s/\Q$SEARCH/$REPLACE/o


John

 

[Anno Siegel]

To the OP:
Uh... Did you test this part? "use constant" happens at compile time,
the assignment to $SEARCH happens later at run time. That would make
KEYSIZE 0 and issue a warning.

Make SEARCH and REPLACE constants too, that would also justify the
caps.

It is not that the data is binary, it is because some of the characters
are special to regular expressions. Use the quotemeta escape to fix it.

s/\Q$SEARCH/$REPLACE/o

That should do it, but with a fixed binary string that size I'd prefer
an index/substr based solution over s///. Something like (untested)

$_ >= 0 and substr( $both, $_, KEYSIZE) = REPLACE for index $both, SEARCH;

Anno